Chris Watts texted Kessinger that his wife took the girls to a play-date and never returned -- but he appeared nonchalant and emotionless, she told the newspaper.

When the media started reporting on the missing persons' case, Kessinger said she was shocked to find out her boyfriend was still married with a wife who was 15 weeks pregnant.

"I thought, 'If he was able to lie to me and hide something that big, what else was he lying about?’" she told the Post.

Kessinger told the paper she peppered her boyfriend with questions but said he would try to change the subject.

Kessinger said she ultimately called investigators about her relationship with Chris Watts.

“I just wanted to help,” she told the newspaper. “With a pregnant woman and two children missing, I was going to do anything that I could.”

Within days of the disappearance, Chris Watts was arrested and the bodies of his wife and children were found.

Kessinger told the newspaper that since his arrest, she has had no doubt that he committed the crimes.

Chris Watts is set to be sentenced Monday after he pleaded guilty to all charges against him: five counts of murder in the first degree; three counts of tampering with a deceased human body; and one count of unlawful termination of pregnancy.

In exchange for the guilty plea, prosecutors will not pursue the death penalty, the Weld County District Attorney's office said. The victims' family agreed to those terms, the district attorney's office added.